1. Field of the Invention
The present invention relates to a method and an apparatus for treating a wastewater containing nitrogen compounds such as organic nitrogen, nitrite nitrogen, nitrate nitrogen and ammonia nitrogen, and phosphorus compounds such as phosphoric acid and phosphate ions (hereinafter, “a wastewater to be treated” will be referred to as “a for-treatment wastewater”).
2. Description of the Related Art
It is already well known that the existence of nitrogen compounds is one of causes of eutrophication of rivers and lakes. The nitrogen compounds much exist in domestic life wastewater or industrial wastewater, but it is difficult to purify them and there are no effective countermeasures up to date. In general, a biological treatment has been implemented. However, the biological treatment comprises two processes, i.e., a nitrification process for converting ammonia nitrogen to nitrate nitrogen, and a denitrification process for converting nitrate nitrogen to a nitrogen gas. Accordingly, there has been a problem that two different reaction vessels are required. There has been a further problem that because a time required for the treatment is long, the treatment efficiency is low.
Further, in this biological treatment, there has been another problem that a large-capacity anaerobic vessel is necessary for keeping nitrifying bacteria and denitrifying bacteria, thereby to induce the increase in equipment construction cost and apparatus installation area. There has been a further problem that the nitrifying bacteria and the denitrifying bacteria are largely influenced by ambient temperature environment, components contained in the for-treatment wastewater, and the like, and in particular, during the winter season when the temperature is low, their activities are lowered to deteriorate the denitrifying action, resulting in an unstable processing efficiency.
Accordingly, there has been proposed a method for solving the foregoing technical problems, wherein a current is fed to the for-treatment wastewater to decompose ammonia, nitrite nitrogen and nitrate nitrogen through oxidation or reduction into a nitrogen gas. In such a conventional electrolytic process for the for-treatment wastewater, a noble metal such as platinum, iridium or palladium is used as an anode.
In this process, a current is allowed to flow through the for-treatment wastewater, so that ammonia nitrogen is oxidized with active oxygen or hypochlorous acid at the anode, and nitrogen compounds are converted into a nitrogen gas, whereby the treatment of the nitrogen compounds is accomplished.
In addition to the above process, there is another process in which iron is used for electrodes constituting an anode and a cathode, and a current is allowed to flow through the for-treatment wastewater, so that ammonia, nitrite nitrogen and nitrate nitrogen are oxidized or reduced to decompose the for-treatment wastewater into a nitrogen gas.
However, in the conventional nitrogen compounds treating method based on the electrolysis, a removal treatment ability of the nitrogen compounds is poor, and hence, it is difficult to treat the nitrogen compounds in the actual treatment of the life wastewater or the industrial wastewater. In addition, nitrate nitrogen scarcely becomes the nitrogen gas, and the removal of nitrate ions at a low concentration is difficult. Consequently, there is also a problem that they remain as a nitrogen component in the wastewater and they cannot be removed.
Furthermore, in the nitrogen treating method by the electrolysis in which the iron electrodes are used, the nitrogen compounds in the for-treatment wastewater are treated by use of iron dissolved in the for-treatment wastewater. Therefore, it is necessary to dissolve a great deal of iron in the for-treatment wastewater, which causes a problem of durability. Moreover, the iron ions dissolved in the for-treatment wastewater brings about a precipitation reaction together with phosphorus compounds present in the for-treatment wastewater, and at this time, they are precipitated in the form of iron phosphate and the like. In this case, a sludge is produced in large quantities from iron phosphate and the like, which causes a problem of its disposal.
Accordingly, the present applicant has already suggested a method in which an alloy of copper and zinc, copper and nickel, or the like is used as a cathode, and a noble metal material such as platinum, iridium or palladium is used as an anode to electrolyze a for-treatment wastewater. According to this treatment method, a reductive reaction of nitrate nitrogen in the for-treatment wastewater into nitrite nitrogen and ammonia is accelerated at the cathode, and ammonia produced at the cathode can bring about a denitrification reaction together with hypochlorous acid produced at the anode. The thus obtainable synergistic effect enables shortening a time taken for the reductive reaction, and treating nitrate ions even at a low concentration.
However, in a case where an alloy containing copper in the cathode is used, harmful copper ions dissolved in the for-treatment wastewater inevitably cause a problem. Therefore, it has been desired to develop a treatment method of the nitrogen compounds without any production of harmful substances.
Furthermore, in the conventional treatment method of the nitrogen compounds by the electrolysis, a surface area of a portion of an electrode constituting the anode which is immersed in the for-treatment wastewater is substantially equal to that of a portion of an electrode constituting the cathode which is immersed in the for-treatment wastewater. In consequence, a reductive reaction of nitrate nitrogen, i.e., nitrate ions into nitrite ions at the cathode is inhibited by an oxidation reaction of nitrite ions into nitrate ions at the anode, whereby a treatment ability of removing the nitrogen compounds inconveniently deteriorates.
The present invention has been developed to solve the above conventional technical problems, and an object of the present invention is to provide a wastewater treating method and a wastewater treating apparatus by which nitrogen compounds are effectively removed without any production of harmful substances, and miniaturization of the apparatus and decrease in costs can be achieved.